The invention relates to the optical transmission of information and, more particularly, to a method and apparatus for compensating for chromatic dispersion that accrues over optical fiber transmission systems.
The availability of high performance optical amplifiers such as the Erbium-Doped Fiber-Amplifier (EDFA) has renewed interest in the use of wavelength division multiplexing (WDM) for optical transmission systems. In a WDM transmission system, two or more optical data carrying channels are combined onto a common path for transmission to a remote receiver. Typically, in a long-haul optical fiber system, the set of wavelength channels would be amplified simultaneously in an optical amplifier based repeater. The Erbium-Doped Fiber-Amplifier is particularly useful for this purpose because of its ability to amplify multiple wavelength channels without crosstalk penalty.
Typically, it is advantageous to operate long-haul transmission systems at high data rates per channel. For example, useful data rates include multiples of the Synchronous Digital Hierarchy (SDH) standard, i.e., 2.5 and 10 Gb/s. As the bit rates increase through the gigabit per second range, the optical powers launched into the transmission fiber need to approach 1 mW per channel. As was demonstrated by Bergano et al. (European Conference on Optical Communications, Brussels, Belgium, paper Th.A.3.1, September 1995) the Non-Return-to-Zero (NRZ) transmission format is particularly useful for transmitting large amounts of data over optically amplified fiber paths. However, NRZ channels operating over long distances require sufficient control over the total amount of chromatic dispersion to ensure low dispersion penalties. Accordingly, the preferred transmission medium for such a system is dispersion shifted optical fibers.
Crosstalk, or the mixing of channels through the slight nonlinearity in the transmission fiber, may arise from the combination of long distance, low dispersion and high channel power. The transmission of many WDM channels over transoceanic distances may be limited by nonlinear interactions between channels, which in turn is affected by the amount of dispersion. This subject was reviewed by Tkach et al. (Journal of Lightwave Technology in Vol. 13, No. 5, May 1995 pp. 841-849). As discussed in Tkach et al., this problem may be overcome by a technique known as dispersion mapping, in which the generation of mixing products is reduced by offsetting the zero dispersion wavelength of the transmission fiber from the operating wavelengths of the transmitter. This technique employs a series of amplifier sections having dispersion shifted fiber spans with either positive or negative dispersion. The dispersion accumulates over multiple fiber spans of approximately 500 to 1000 km. The fiber spans of either positive or negative sign are followed by a dispersion-compensating fiber having dispersion of the opposite sign. This subsequent section of fiber is sufficient to reduce the average dispersion (averaged over the total length of the transmission system) substantially to zero. That is, a fiber of high negative (positive) dispersion permits compensation by a length of positive (negative) transmission fiber.
The efficacy of the dispersion mapping technique is limited because the amount of dispersion that occurs in a typical optical fiber depends on the operating wavelength that is employed. That is, only one given wavelength can operate at average zero dispersion. The wavelength dependence of the dispersion coefficient is sometimes referred to as the dispersion slope of the fiber. Accordingly, because of this characteristic of the dispersion slope, the various channels employed in a WDM system cannot all operate at the wavelength of average zero dispersion. This limitation can be overcome to a limited degree by using individual channel dispersion compensation at the receiver. However, since these systems are subject to nonlinear penalty, the ability to correct for the non-zero dispersion at the receiver terminal is limited.
One method and apparatus for managing dispersion in a WDM optical transmission system is shown in U.S. application Ser. No. 08/759,493. In this reference the usable optical bandwidth of the transmission system is divided into sub-bands that individually undergo dispersion compensation before being recombined. Accordingly, in comparison to other dispersion mapping techniques, more WDM data channels reside near a wavelength corresponding to the average zero dispersion wavelength. Unfortunately, this arrangement makes it difficult to upgrade the transmission system by adding more channels since the splitting/recombining elements that produce the sub-bands are designed for particular wavelengths and thus the channel wavelengths can only be changed by replacing the splitting/recombining elements.
Accordingly, there is a need for a dispersion compensator for an optical transmission system that allows the system to be upgraded by adding more channels.
In accordance with the method of the present invention, dispersion compensation is provided to a WDM optical signal having a plurality of channels located at different wavelengths and traveling in an optical transmission path. The method begins by selecting a subset of the plurality of channels that fall below a prescribed performance threshold when no dispersion compensation is performed. Next, dispersion compensation is provided to each of the subset of channels without compensating for dispersion in the remaining channels.
Since the present invention only provides dispersion compensation to a subset of channels, system upgrading can be conveniently performed in those wavebands corresponding to the remaining channels that do not undergo dispersion compensation.
In accordance with one aspect of the invention, the prescribed performance threshold is a Q-value for the optical transmission system. Since in some transmission systems the Q-value is relatively high over a wide bandwidth that encompasses all but the outermost channels, the selected subset of channels may be limited to the outermost channels. In such a case system upgrading can be performed over the wide bandwidth region that encompasses all but the outermost channels.
In accordance with another aspect of the invention, a dispersion compensator provides dispersion compensation to a WDM optical signal having a plurality of channels located at different wavelengths and traveling in an optical transmission path. The dispersion compensator includes an optical splitter adapted to receive the WDM optical signal. The optical splitter has first and second output ports such that a subset of the plurality of channels are directed along the first output port and remaining ones of the plurality of channels are directed along the second output port. A dispersion compensating element is coupled to the first output port and a multiplexing element having a first input port is coupled to second output port of the optical splitter. The multiplexing element also has a second input port coupled to the dispersion compensating element and an output port on which the subset of channels and the remaining ones of the channels are recombined.
In accordance with another aspect of the invention, the optical splitter includes an optical circulator. The optical splitter may also include at least one fiber Bragg grating coupled to a port of the circulator and reflecting thereto the subset of the plurality of channels that are to undergo dispersion compensation.
In accordance with yet another aspect of the invention, the multiplexing element is an optical circulator.
In accordance with another aspect of the invention, the optical splitter includes a wavelength routing device.
In accordance with another aspect of the invention, the dispersion compensator may be a single-mode optical fiber or a fiber Bragg grating.